1. Field of the Invention
This invention relates to a punch press; and more particularly to one in which utilizes piezo-electric actuators as punch driving sources to punch a workpiece, such as a metal foil.
2. Discussion of the Related Art
A punching press for shearing a workpiece, such as a thin metal foil (several tens of micrometers) with high accuracy, namely, a press with lamination type piezo-electric actuators as punch driving sources is disclosed Japanese Patent Application (OPI) No. 127997/1990 (the term "OPI" as used herein means an "unexamined published application").
FIG. 10 shows the arrangement of the aforementioned press. As shown in FIG. 10, a frame 1 incorporates a variety of components such as a die unit comprising an upper die set 2, lower die set 3, movable stripper 4, upper die 5, lower die 6, upper punch 7 and lower punch 8, and piezo-electric actuators 9 for operating the stripper 4. Two actuator casings 12 are disposed on the top and bottom of the frame 1, respectively. The actuator casings 12 include piezo-electric actuators 10 provided as drive sources for the upper punch 7 and the lower punch 8 in combination with movable pieces 11, which are rod members for transmitting the amount of displacement of the respective piezo-electric actuators 10 to the punches. Further in FIG. 10, reference numeral 13 designates tightening screws for fixedly securing the die sets 2 and 3 inside the frame; springs 14 for urging the movable stripper 4; springs 15 for returning the punches 7 and 8; and pre-loaded springs 16 mounted on the movable pieces 11 so as to maintain pressure on the piezo-electric actuators at all times, the springs 16 being incorporated in the casings 12 together with the movable pieces 11.
In order to punch a belt-shaped workpiece or a metal foil 17, the press thus constructed operates as follows:
First, the piezo-electric actuators 9 are operated to move the movable stripper 4 upwardly. Under this condition, the workpiece 17 is fed into the space between the upper die 5 and the lower die 6. Thereafter, the piezo-electric actuators 9 are restored so that the-workpiece 17 is held between the upper die 5 and the lower die 6 with the aid of the elastic forces of the urging springs 14. Under this condition, the upper punch 7 and the lower punch 8 are driven alternately with the piezo-electric actuators 10, to punch the workpiece 17 by so-called "vertical two-way punching". Thereafter, the piezo-electric actuators 9 are operated again to move the stripper 4 thereby to release the workpiece. The workpiece 17 thus released is shifted a predetermined distance.
The above-described press is impractical. As described above, in the press of FIG. 10, the piezo-electric actuators 10 are fixedly mounted on the top and the bottom of the frame 1, respectively, and the piezo-electric actuators 9 are driven to move the movable stripper 4 adapted to hold the workpiece 17. In general, the amount of displacement provided by a lamination type piezo-electric actuator, which is formed by stacking a number of piezo-electric elements, is no more than several tens of microns (.mu.m). Therefore, when the movable stripper 4 is moved upwardly as was described above, the space formed between the upper and lower dies of the press is such that the metal foil workpiece 17 can be barely inserted into it; that is, the space is not quite large enough.
For example, if the workpiece is bent during pressing, or burrs are formed at the cut edges during shearing, the workpiece may be caught in the press. In such event, sometimes, it is at times impossible to feed the workpiece. If this occurs, no emergency measure can be taken; that is, it is impossible to release the workpiece, for instance, with a jig or tool inserted from outside, because the space between the upper and lower dies is considerably narrow, of the order of several tenths of microns. Therefore, the die unit must be disassembled to remove the workpiece caught in the press, which is a troublesome operation.